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# PT93.S2.Q7 - Mu mesons generated cosmic rays

Member
edited November 2022 82 karma

Can someone break this one down for me? I am really struggling to understand how D is correct.

• Member
772 karma

Okay so for this question its important to keep track of all the comparative language. I'm going to use a lot of paraphrasing into simple language which is what I did during my timed run.

They first introduce the little buggers called Mu Mesons (let's just call them Mesons from here on out). So we know that the Mesons generated by cosmic rays which are just outside of Earth's atmosphere are travelling at speeds approaching the speed of light. Great, let's see what else they have for us...

We then get a contrast with the Mesons generated in the laboratory (as opposed to generated by cosmic rays). They tell us that these Lab Mesons decay in much less time than it takes for Mesons to travel from our atmosphere to our detection apparatus (literally just picture a radio telescope or some other device that we have on ground to measure the Mesons that are hitting our planet).

Let's make up an example using easy numbers to understand this comparison. Let's say that the cosmic ray mesons usually take 10 seconds to travel through our atmosphere to then be detected by our devices. Accoring to the stimulus, the Lab Mesons decay in MUCH LESS time than these 10 seconds, so let's just say 2 seconds. (can you see where this is going?? if not, its okay, let's keep reading).

It then says that If the Mesons traveling through our atmosphere were decaying as fast as our Lab Mesons (aka 2 seconds instead of 10 seconds), then we would only be able to measure 1/100 of the number we actually do. So basically, we wouldn't get as many readings as we do now because they would be decaying too quickly and not reaching our device.

Now, they ask you to logically complete the stimulus.

So they told us that IF (key word IF) the Cosmic Ray Mesons we are measuring were decaying as fast as our Lab Mesons then we would only measure a tiny tiny portion of what we actually measure, what does that mean??? Well, it has to mean that the Cosmic Ray Mesons ARE NOT decaying at that rate. They must be decaying much slower, otherwise how are we still detecting so many of them.

D says just that. It says those Mesons moving at the speed of light (which are our cosmic ray mesons) decay more slowly than our Lab Mesons (aka the Mesons almost at rest).

Hope that helps. That's kind of the intuitive way of getting there.

Using conditional logic, you could use the last two sentences in the following manner:

IF those Mesons travelling at the speed of light decay as fast as they do in a lab -----> we would only detect 1/100 of them.

But since we are not just detecting 1/100 of them, you can contrapose to conclude that they must not decay as fast as they do in a lab which means that they decay more slowly which again is what D says.

I think a simplified version of the argument would be something like.

If I was a slow runner, then i wouldnt have won the race. But i did win the race. so i must not be a slow runner.

I actually didn't notice it was this simple until now lol, I just translated every bit slowly as I went on and then intuitively got there during my timed run.

• Member
82 karma

@Steven_B-1

Wow... that clicked. Thank you so so much. I was staring at this question for so long and couldn't get past all of the jargon. Really really appreciate it.

• Member
772 karma

My pleasure !

13839 karma

Great explanation @Steven_B-1

• Member
772 karma

Thanks JY !

• Alum Member
3 karma

Hi, Super late to the party, but I have read and re-read this including the explanation, but I am still 100% confused.

In your explanation you mention "But since we are not just detecting 1/100 of them"; Why would we think that we are not detecting 1/100 when the passage says just that?

I hope you see this and help out! Thank you so so much